1. Field of the Invention
This invention relates to a system for high-precision measurement of the distance among a plurality of signal reception points or between artificial satellites and signal reception points using the correlation between two spread spectrum wave signals.
2. Prior Art Statement
As a method for measuring position using the GPS (Global Positioning System) satellite under development by the United States there has been developed a system in which the receiver has a built-in code generator which uses the 1575.42 MHz (hereinafter referred to as L1) band distance-measurement spread signal whose code data has been made available to general users (hereinafter referred to as the C/A code). However, the spread modulation signals (hereinafter referred to as the P code) for high-precision position measurement that are transmitted on L1 and the 1227.6 MHz (hereinafter referred to as L2) band have been designated for military use and the code data is not available for general users. It is therefore generally difficult to carry out high-precision positioning using the P code.
On the other hand, development is proceeding on distance measurement systems that use signals transmitted by the GPS satellite for high-precision relative position measurement and do not require code data. These systems fall into three categories:
(1) A system in which the received signal is squared to reconstruct a continuous signal of twice the frequency of the carrier wave; PA1 (2) A system in which the spread modulation clock signal of the received signal is reconstructed; PA1 (3) A system in which the above two systems are used in combination.
These systems determine range by measuring the phase (time interval relative to the reference clock at the reception point) of the reconstructed carrier wave signal or clock signal at the time of reception. The time difference between the clock aboard the satellite and the clock at the reception point and the product obtained by multiplying the time delay caused by the intervening ionosphere and atmosphere along the propagation path by the speed of light are included in the determined distance. The result is thus called the "pseudo-range" as opposed to the "true range." The measurement is simultaneously conducted at two or more reception points with respect to four or more satellites and the distances among (relative positions of) the reception points are calculated from the results of the measurement.
FIG. 1(a) shows a conventional GPS receiver system for precision relative position measurement which will be used as the basis for explaining the principle involved up to the determination of the pseudo-range by reconstruction of the carrier wave or the spread clock signal from the spread spectrum signal. Signals of frequency L1 and L2 received by an antenna 1 are converted to an intermediate frequency in frequency converters 2, 3, respectively, and a carrier signal and a clock signal are reconstructed in carrier or clock reconstructors 4, 5. Next a pseudo-range including ambiguity arising during each period of the reconstructed signals is obtained by forwarding the signals reproduced by the reconstructors 4, 5 to reception time detectors 6, 7 and measuring them with reference to a reference clock 9 at the reception time. The measured data is forwarded to a data processing computer 8 where it is stored. The ambiguity arising during each period of the reconstructed signals is then removed from the stored data to obtain a pseudo-range.
The principle of the carrier reconstructors 4, 5 of FIG. 1(a) will now be explained with reference to FIG. 1(b). The received spread signal P(t) cos(.omega.t) is split and the two resulting signals are multiplied in a multiplier 10, whereby there is reconstructed a continuous wave signal having twice the frequency of the carrier frequency. The result is forwarded to a band pass filter 11 for extracting only a signal cos(2 .omega.t).
The principle of the clock reconstructors 4, 5 (for spread modulation) of FIG. 1(a) will next be explained with reference to FIG. 1(c). The received spread signal P(t)cos(.omega.t) is split and one of the two resulting signals is forwarded directly to a multiplier 13, while the other is sent to a 1/2-clock delay circuit 12 where it is delayed by 1/2 period of the spread clock and is then forwarded to the multiplier 13. The multiplier multiplies the two signals and the resulting reconstructed clock signal is passed through a band pass filter 14 to extract a spread clock signal. These signal reconstruction methods are widely used for carrier wave reconstruction and clock reconstruction of PSK (phase shift keying) signals in digital communication.
In the carrier wave reconstruction system (1) mentioned above, the period of the reconstructed carrier signal is short so that from the point of ambiguity elimination for calculating the pseudo-range from the measured results, a prolonged time and data processing time are required, which places a large burden on the software.
In the spread modulation clock signal reconstruction system (2), the long clock period makes removal of ambiguity relatively easy but for increasing the precision of the measurement it is necessary either to conduct measurement over a long period of time or to use an antenna with large gain.
Moreover, measurement is conducted only at a single frequency in these systems, making it impossible to measure the propagation delay time caused by the ionosphere, which, having an effect inversely proportional to the square of the frequency of the carrier wave, is one cause of error. Another problem is that it is necessary to conduct the measurement in both the L1 and L2 frequency bands and then to estimate the ionospheric propagation delay from the difference between the results obtained. The receiver is thus required to have separate systems for L1 and L2. Not only is this uneconomical, it also complicates the data processing.
The object of this invention is thus to provide, in a high-precision relative position measuring system for receiving signals from a plurality of GPS satellites and measuring pseudo-ranges between the satellites and the reception point based on the received signals, an improved ranging system using the correlation between two spread spectrum wave signals which is capable of calculating pseudo-range easily and with high precision using only a single signal processing system.